(1) Field of the Invention
The present invention relates to an optical system of a rear projection picture display apparatus for displaying a large picture by projecting the picture displayed on a picture display unit using a projection lens onto a screen.
(2) Description of the Prior Art
A picture display apparatus having high resolution and a large screen has been desirable for commercial uses such as High Vision and industrial uses such as high grade computer displays. With the above applications, projection type picture display apparatus are spreading throughout the market.
Projection type picture display apparatus are classified into two types. One is a two unit type which projects a picture on a screen which is separated from the projector unit. Another is a single unit type in which a projector unit and a screen are built in one enclosure. The latter is called rear projection type.
The former is considerably small, has minimal restrictions in projection distance and is free in projection size. However, if external light comes on the screen, the projected picture is accompanied by black buoying. Therefore, it is desirable to use the former in a dark room (or in a similar type of environment). The latter can display a good picture even under a bright environment because no external light from the projector side comes to the screen. However, the apparatus may require a space corresponding to the projection distance. Thus, the enclosure of the apparatus may be big. This is not desirable for home use.
To make an apparatus small size, two kinds of solutions have been considered in a rear projection type. One solution is to use a projection lens having a short focal distance in order to reduce the projection distance. Another solution is to miniaturize a projection tube, a projection lens, etc. However, after much experimentation in these areas, it is difficult to expect further progress in the future although a small improvements may be made.
One attempt to make the apparatus size small is disclosed in Japanese Patent Laid-Open 2-140792. An optical system for a rear projection picture display apparatus in accordance with the prior art is shown in FIG. 1. The apparatus includes a picture display unit (not shown in the drawing), a beam splitter 2 for partially reflecting and partially transmitting the incident light, a mirror 3 for returning the reflected light from the beam splitter 2 toward the beam splitter 2 again, a louver 4 provided at the rear surface of the beam splitter 2 so that the light in the previously designated range of the incident angle against the perpendicular line to the beam splitter 2 can transmit to the beam splitter 2, and a screen 5.
With the above-mentioned construction, space for the folded light path of the mirror is reduced.
However, because the louver 4 splits transmitting light and not-transmitting light according to its incident angle, the incident angle differs at the center and at the periphery of the screen, and brightness unevenness occurs unless a constant amount of light is transmitted for any incident angle. Further, because the louver 4 has plural light paths, the transmission factor may decrease, and a moire due to a mutual relation with a displayed picture may appear. As a result, picture quality deterioration will be induced.